Compressors may be used in a wide variety of industrial and residential applications to circulate refrigerant within a refrigeration, heat pump, HVAC, or chiller system (generically “refrigeration systems”) to provide a desired heating or cooling effect. In any of the foregoing applications, the compressor may be used in conjunction with a capacity modulation system that adjusts a capacity of the compressor based on system demand.
Conventional capacity modulation systems selectively adjust the ability of the compressor to circulate refrigerant through the refrigeration system and therefore adjust the ability of the refrigeration system to absorb and reject heat. Conventional capacity modulation systems may therefore be used to adjust a capacity of the refrigeration system based on a required heating and/or cooling demand. Regulating compressor capacity based on system demand improves the efficiency of the compressor as only that amount of energy that is required is consumed.
Conventional capacity modulation systems may adjust compressor capacity by regulating a pressure within a compressor housing to prevent operation of a compression chamber disposed within the housing. For example, in a scroll compressor application, a conventional capacity modulation system may permit a non-orbiting scroll member to separate from an orbiting scroll member. Such separation creates a leak path between the non-orbiting scroll member and the orbiting scroll member and therefore reduces the ability of the compressor to compress refrigerant.
Leak paths may be accomplished by exposing the non-orbiting scroll member to low-pressure vapor (i.e., vapor at suction pressure) or to intermediate-pressure vapor or high-pressure vapor (i.e., vapor at discharge presue) through actuation of a valve. Pulse width modulation may be used to cycle the valve between an open state and a closed state to achieve a desired capacity of the compressor. Typically, the valve is cycled at a rate such that the valve is closed when the compressor is loaded and is open when the compressor is unloading.
During loading of the compressor, suction pressure at an inlet of the compressor steadily decreases, while during unloading of the compressor, suction pressure steadily increases. The decrease in suction pressure over time results in a reduction in capacity as the compressor is required to consume additional energy to compress the low-pressure vapor to discharge pressure when compared to the energy consumed in compressing vapor at a higher pressure (i.e., earlier during loading of the compressor). Therefore, the efficiency of the compressor decreases with decreasing suction pressure.